The electromagnetic turbulence and associated frequencies observed far upstream of comet P/Grigg-Skjellerup during the recent Giotto spacecraft flyby can be attributed to relative streaming between the cometary ions and the solar wind, and the hypothesis that unstable parallel modes are responsible is used to deduce the wavelengths involved. Such a self-consistent approach yields resonant instabilities with wavelengths of 7000~8000 km, phase velocities which are sub-Alfv¿nic and polarizations which are left-handed in the spacecraft frame. ¿ American Geophysical Union 1993